1. Field
This application relates generally to position sensing probes and, more specifically, to position sensing probes capable of sensing position simultaneously in two axes of a plane defined by cartesian coordinates.
2. State of the Art
Position information concerning various characteristics of a manufactured workpiece is important to quality control of the workpiece. Inspection of a characteristic, such as an aperture in a surface, is frequently accomplished with the use of a coordinate measuring machine (CMM). The CMM can be numerically programed to measure sequentially the position of many characteristics on a workpiece. The inspection data can then be used to verify machine setups or to monitor quality control.
The probe which extends from the CMM to communicate positional information can be either the contact type, such as a stylus, or the non-contact type, such as a laser sensing device. The contact-type probe must relay the contact event to a sensing device while, in many cases, the non-contact probe is itself a sensing device.
Sensors are typically located so that each sensor receives positional information relating to some aspect of the system defining the position. For example, if an aperture is located in a surface of a workpiece according to coordinates in a cartesian coordinate volume, a sensor would be required on each of the X, Y, and Z axes defining the coordinate system.
Several numerically controlled CMM's are capable of determining X, Y, and Z coordinates. These machines are sophisticated devices utilizing computer control of servomotors, sliding rail systems for probe movement in the relevant axes, and data acquisition and storage. The machines are expensive, large, and difficult to move and set up.
A contact sensing probe that produces an output signal when the probe's stylus engages a workpiece is disclosed in U.S. Pat. No. 4,854,050 to McMurtry. The device is used to measure the dimensions of a workpiece while supported by a fixed machine. The invention in McMurtry utilizes a probe stylus in communication with a strain sensitive element serving as the sensing means. When the probe stylus contacts the workpiece, a shock wave generated by the contact is transmitted to the strain sensitive element which produces an output signal in the form of a voltage potential. The output potential is routed to an opto-electronic system which records the position on a track where the stylus contacted the workpiece. A touch probe which similarly measures linear dimensions by using positional sensors is disclosed in U.S. Pat. No. 4,553,332 to Galinelli, et al.
The contact sensing probes in McMurtry and Galinelli cannot simultaneously determine the probe's position in both axes of a cartesian coordinate plane unless a second track system is installed. This expensive, and often impractical, solution to dual axes location of the probe is a limiting factor imposed on the devices.
A sensing device to locate the rotational axis of an irregularly shaped object so that the axis is coaxial with a workpart chuck axis is disclosed in U.S. Pat. No. 4,790,545 to Dzewaltowski, et al. Though the invention in Dzewaltowski is capable of locating the position of a spatial characteristic on the workpiece (the center of rotation), the device is incapable of locating the position of any other characteristic.
A probe for measuring deviations from linearity of straight tubular passages is disclosed in U.S. Pat. No. 4,651,436 to Gaal. The invention in Gaal uses sensors to generate electrical signals when the probe encounters deviations from linearity (distortions) in a tubular passage.
The invention in Gaal is incapable, however, of locating the position of a characteristic on a plane defined by cartesian coordinates.
Several non-contact position sensing devices are capable of determining the position of a workpiece characteristic in relation to a defined area. A non-contact position and dimension sensing device utilizing laser optics is disclosed in U.S. Pat. No. 4,131,365 to Pryor. The invention in Pryor senses the diffraction wave emitted from an object after an incident source of electromagnetic radiation has been directed on the object. The diffraction wave impinges on a detector which outputs a signal that is translated into the particular measurement of interest. U.S. Pat. No. 4,662,752 to Tucker et al. discloses a box with a series of light sensors and beams. The invention in Tucker measures position and orientation of an object relative to a known position and orientation.
To operate the invention in Pryor, a substantial amount of specialized knowledge and electronic equipment is required. Further, in order to determine the position of a characteristic in two axes simultaneously, the electronic equipment must be doubled. The substantial set-up time and effort required between different types of characteristic measurement would make the device cumbersome for more than one specific application. Tucker similarly requires many expensive sensors and is limited to objects which fit into the box. Tucker also appears to be limited to measurements of objects rather than characteristics.
A coordinate measuring machine capable of measuring generally cylindrical objects is disclosed in U.S. Pat. No. 4,800,652 to Ballas, et al. The machine in Ballas is very large and expensive and determines the position of a characteristic by a different method than the present invention. Ballas utilizes multiple probes and a unique coordinate system only compatible with generally cylindrical objects.
Touch probes having position sensors on one or more axes defining the position of a characteristic are known in the art. A touch probe operable in two axes for gauging and indication applications is disclosed in U.S. Pat. No. 4,542,590 to Cusack. The probe in Cusack has a stylus extending from a body which contains mechanical sensing means and electrical sensors. The sensing means is movable within a small range of motion and held in a rest position by a series of reed and wire springs. Movement of the probe stylus produces a concomitant movement in the sensing means which is detected by several electrical contacts. The design of the probe in Cusack precludes simultaneous three-axis positional information.
A three-axis touch probe is disclosed in U.S. Pat. No. 4,937,948 to Herzog, et al. Three-axis movement is made possible by flexible leaf springs in a Hooke's joint. Deflection of the probe stylus from the rest position is sensed by measurement grids in an incremental distance measuring system.